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UART stands for Universal Asynchronous Receiver-Transmitter. It is a type of hardware communication protocol that facilitates serial communication between electronic devices. UART is commonly used to transmit and receive data between a microcontroller or a computer and peripheral devices such as sensors, actuators, displays, and other components.


UART communication involves sending and receiving individual bits of data sequentially over two wires: a transmit (TX) wire and a receive (RX) wire. The communication is asynchronous, meaning that there is no common clock signal shared between the transmitting and receiving devices. Instead, both devices agree on a common baud rate (data transfer rate) to synchronize the communication.

The basic characteristics of UART communication include:

  • Start Bit: Each data frame starts with a low-level start bit, indicating the beginning of the data transmission.
  • Data Bits: The actual data to be transmitted or received, usually ranging from 5 to 9 bits per frame.
  • Parity Bit (optional): A parity bit can be included to help detect errors in the transmission. It can be set to odd, even, mark, space, or none.
  • Stop Bit(s): One or more high-level stop bits indicate the end of a data frame. The stop bit(s) provide time for the receiver to prepare for the next data frame.


  • GND - Ground connection, required for good TX/RX.
  • TX - This is the transmission line.
  • RX - This is the receiving line.
  • VCC - Connection power (generally left unconnected).

Baud Rate

Baud rate, often denoted as "baud," is a crucial parameter in serial communication systems like UART (Universal Asynchronous Receiver-Transmitter). It determines the rate at which data is transmitted over a communication channel in terms of signal changes per second. In other words, the baud rate specifies how quickly the signaling voltage levels on the communication line change to represent data.

It's important to note that baud rate is not the same as bits per second (bps), although the terms are sometimes used interchangeably. Baud rate refers to the number of signaling events (changes in voltage levels) that occur in a second, while bits per second refers to the number of actual data bits transmitted per second.

For example, in a UART communication system with a baud rate of 9600 baud, one signaling event might represent multiple bits. If the system uses 8-N-1 configuration (8 data bits, no parity, 1 stop bit), then a single character would consist of 10 bits (8 data bits + 1 start bit + 1 stop bit). Therefore, the actual data rate in bits per second (bps) would be 9600 / 10 = 960 bps.

Flow Control

Flow control in serial communication is a mechanism used to manage the data flow between a sender and a receiver to prevent data loss or buffer overflow when the receiving device is temporarily unable to process incoming data. It ensures that the sender doesn't overwhelm the receiver with data at a faster rate than it can handle. Flow control becomes especially important when the sender and receiver have different processing speeds or capacities.

There are two main types of flow control in serial communication: hardware flow control and software (also known as XON/XOFF) flow control.

Hardware Flow Control

Hardware flow control involves using dedicated control lines (wires) for managing the data flow. The most common hardware flow control signals are:

  • Request to Send (RTS): The sender asserts this signal to indicate that it is ready to transmit data.
  • Clear to Send (CTS): The receiver asserts this signal to indicate that it is ready to receive data.
  • Data Terminal Ready (DTR): The sender asserts this signal to indicate that it is ready for communication.
  • Data Set Ready (DSR): The receiver asserts this signal to indicate that it is ready for communication.

When a sender (transmitter) wants to send data, it checks the state of the CTS signal before transmitting. If CTS is asserted by the receiver, it means the receiver is ready to receive data. If not, the sender waits until CTS is asserted.

Software Flow Control (XON/XOFF)

Software flow control involves using special control characters to signal the sender to stop or resume transmission. This method doesn't require dedicated control lines; instead, it's integrated into the data stream. The two control characters used are:

  • XON (Transmit On): When the receiver's buffer is ready to receive data, it sends an XON character to the sender to indicate that transmission can continue.
  • XOFF (Transmit Off): If the receiver's buffer is nearing its capacity, it sends an XOFF character to the sender to indicate that transmission should be paused until further notice.

The sender monitors for XOFF characters and stops transmitting when it receives one. It resumes transmission when it receives an XON character.

Hardware flow control is considered more reliable and efficient than software flow control, especially in noisy environments. However, software flow control is simpler to implement and doesn't require extra hardware connections. The choice between the two depends on the specific requirements of the communication system and the devices involved.


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